Request For Comments - RFC5684

Independent Submission P. Srisuresh
Request for Comments: 5684 EMC Corporation
Category: Informational B. Ford
ISSN: 2070-1721 Yale University
February 2010
Unintended Consequences of NAT Deployments
with Overlapping Address Space
Abstract
This document identifies two deployment scenarios that have arisen
from the unconventional network topologies formed using Network
Address Translator (NAT) devices. First, the simplicity of
administering networks through the combination of NAT and DHCP has
increasingly lead to the deployment of multi-level inter-connected
private networks involving overlapping private IP address spaces.
Second, the proliferation of private networks in enterprises, hotels
and conferences, and the wide-spread use of Virtual Private Networks
(VPNs) to access an enterprise intranet from remote locations has
increasingly lead to overlapping private IP address space between
remote and corporate networks. This document does not dismiss these
unconventional scenarios as invalid, but recognizes them as real and
offers recommendations to help ensure these deployments can
function without a meltdown.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any
other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value
for implementation or deployment. Documents approved for
publication by the RFC Editor are not a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5684.
Srisuresh & Ford Informational [Page 1]

RFC 5684 Complications from NAT Deployments February 2010
1. Introduction and Scope
The Internet was originally designed to use a single, global 32-bit
IP address space to uniquely identify hosts on the network, allowing
applications on one host to address and initiate communications with
applications on any other host regardless of the respective host's
topological locations or administrative domains. For a variety of
pragmatic reasons, however, the Internet has gradually drifted away
from strict conformance to this ideal of a single flat global address
space, and towards a hierarchy of smaller "private" address spaces
[RFC1918] clustered around a large central "public" address space.
The most important pragmatic causes of this unintended evolution of
the Internet's architecture appear to be the following.
1. Depletion of the 32-bit IPv4 address space due to the exploding
total number of hosts on the Internet. Although IPv6 promises to
solve this problem, the uptake of IPv6 has in practice been slower
than expected.
2. Perceived Security and Privacy: Traditional NAT devices provide a
filtering function that permits session flows to cross the NAT in
just one direction, from private hosts to public network hosts.
This filtering function is widely perceived as a security benefit.
In addition, the NAT's translation of a host's original IP
addresses and port number in a private network into an unrelated,
external IP address and port number is perceived by some as a
privacy benefit.
3. Ease-of-Use: NAT vendors often combine the NAT function with a
DHCP server function in the same device, which creates a
compelling, effectively "plug-and-play" method of setting up small
Internet-attached personal networks that is often much easier in
practice for unsophisticated consumers than configuring an IP
subnet. The many popular and inexpensive consumer NAT devices on
the market are usually configured "out of the box" to obtain a
single "public" IP address from an ISP or "upstream" network via
DHCP ([DHCP]), and the NAT device in turn acts as both a DHCP
server and default router for any "downstream" hosts (and even
other NATs) that the user plugs into it. Consumer NATs in this
way effectively create and manage private home networks
automatically without requiring any knowledge of network protocols
or management on the part of the user. Auto-configuration of
private hosts makes NAT devices a compelling solution in this
common scenario.
[NAT-PROT] identifies various complications with application
protocols due to NAT devices. This document acts as an adjunct to
[NAT-PROT]. The scope of the document is restricted to the two
Srisuresh & Ford Informational [Page 3]

RFC 5684 Complications from NAT Deployments February 2010
scenarios identified in sections 3 and 4, arising out of
unconventional NAT deployment and private address space overlap.
Even though the scenarios appear unconventional, they are not
uncommon to find. For each scenario, the document describes the
seeming anomalies and offers recommendations on how best to make the
topologies work.
Section 2 describes the terminology and conventions used in the
document. Section 3 describes the problem of private address space
overlap in a multi-level NAT topology, the anomalies with the
topology, and recommendations to address the anomalies. Section 4
describes the problem of private address space overlap with remote
access Virtual Private Network (VPN) connections, the anomalies with
the topology, and recommendations to address the anomalies. Section
5 describes the security considerations in these scenarios.
2. Terminology and Conventions Used
In this document, the IP addresses 192.0.2.1, 192.0.2.64,
192.0.2.128, and 192.0.2.254 are used as example public IP addresses
[RFC5735]. Although these addresses are all from the same /24
network, this is a limitation of the example addresses available in
[RFC5735]. In practice, these addresses would be on different
networks.
Readers are urged to refer to [NAT-TERM] for information on NAT
taxonomy and terminology. Unless prefixed with a NAT type or
explicitly stated otherwise, the term NAT, used throughout this
document, refers to Traditional NAT [NAT-TRAD]. Traditional NAT has
two variations, namely, Basic NAT and Network Address Port Translator
(NAPT). Of these, NAPT is by far the most commonly deployed NAT
device. NAPT allows multiple private hosts to share a single public
IP address simultaneously.
3. Multi-Level NAT Network Topologies
Due to the pragmatic considerations discussed in the previous section
and perhaps others, NATs are increasingly, and often unintentionally,
used to create hierarchically interconnected clusters of private
networks as illustrated in figure 1 below. The creation of multi-
level hierarchies is often unintentional, since each level of NAT is
typically deployed by a separate administrative entity such as an
ISP, a corporation, or a home user.
Srisuresh & Ford Informational [Page 4]

RFC 5684 Complications from NAT Deployments February 2010
as instructed by the ISP, and then plugging his hosts into the NAT as
the NAT's manual indicates, seems to work and gives all of his hosts
access to Internet.
CheapoISP, an inexpensive service provider, has allocated only one or
a few globally routable IP addresses, and uses NAT to share these
public IP addresses among its many customers. Such an arrangement is
becoming increasingly common, especially in rapidly developing
countries where the exploding number of Internet-attached hosts
greatly outstrips the ability of ISPs to obtain globally unique IP
addresses for them. CheapoISP has chosen the popular 10.1.1.x
address for its private network, since this is one of the three well-
known private IP address blocks allocated in [RFC1918] specifically
for this purpose.
Of the three incentives listed in section 1 for NAT deployment, the
last two still apply even to customers of ISPs that use NAT,
resulting in multi-level NAT topologies as illustrated in the right
side of the above diagram. Even three-level NAT topologies are known
to exist. CheapoISP's customers Ann, Mary, and Lex have each
obtained a single IP address on CheapoISP's network (Private Network
2), via DHCP. Mary attaches only a single host at this point, but
Ann and Lex each independently purchase and deploy consumer NATs in
the same way that Bob did above. As it turns out, these consumer
NATs also happen to use 10.1.1.x addresses for the private networks
they create, since these are the configuration defaults hard-coded
into the NATs by their vendors. Ann and Lex probably know nothing
about IP addresses, and in particular they are probably unaware that
the IP address spaces of their own private networks overlap not only
with each other but also with the private IP address space used by
their immediately upstream network.
Nevertheless, despite this direct overlap, all of the "multi-level
NATed hosts" -- F, G, H, and I in this case -- all nominally function
and are able to initiate connections to any public server on the
public Internet that has a globally routable IP address. Connections
made from these hosts to the main Internet are merely translated
twice: once by the consumer NAT (NAT-3 or NAT-44) into the IP address
space of CheapoISP's Private Network 2 and then again by CheapoISP's
NAT-2 into the public Internet's global IP address space.
3.1. Operational Details of the Multi-Level NAT Network
In the "de facto" Internet address architecture that has resulted
from the above pragmatic and economic incentives, only the nodes on
the public Internet have globally unique IP addresses assigned by the
official IP address registries. IP addresses on different private
networks are typically managed independently -- either manually by
Srisuresh & Ford Informational [Page 6]

RFC 5684 Complications from NAT Deployments February 2010
the administrator of the private network itself, or automatically by
the NAT through which the private network is connected to its
"upstream" service provider.
By convention, nodes on private networks are usually assigned IP
addresses in one of the private address space ranges specifically
allocated to this purpose in RFC 1918, ensuring that private IP
addresses are easily distinguishable and do not conflict with the
public IP addresses officially assigned to globally routable Internet
hosts. However, when plug-and-play NATs are used to create
hierarchically interconnected clusters of private networks, a given
private IP address can be and often is reused across many different
private networks. In figure 1 above, for example, private networks
1, 2, 3, and 4 all have a node with IP address 10.1.1.10.
3.1.1. Client/Server Communication
When a host on a private network initiates a client/server-style
communication session with a server on the public Internet, via the
server's public IP address, the NAT intercepts the packets comprising
that session (usually as a consequence of being the default router
for the private network), and modifies the packets' IP and TCP/UDP
headers so as to make the session appear externally as if it were
initiated by the NAT itself.
For example, if host C above initiates a connection to host A at IP
address 192.0.2.64, NAT-1 modifies the packets comprising the session
so as to appear on the public Internet as if the session originated
from NAT-1. Similarly, if host F on private network 3 initiates a
connection to host A, NAT-3 modifies the outgoing packet so the
packet appears on private network 2 as if it had originated from
NAT-3 at IP address 10.1.1.10. When the modified packet traverses
NAT-2 on private network 2, NAT-2 further modifies the outgoing
packet so as to appear on the public Internet as if it had originated
from NAT-2 at public IP address 192.0.2.254. The NATs in effect
serve as proxies that give their private "downstream" client nodes a
temporary presence on "upstream" networks to support individual
communication sessions.
In summary, all hosts on the private networks 1, 2, 3, and 4 in
figure 1 above are able to establish a client/server-style
communication sessions with servers on the public Internet.
3.1.2. Peer-to-Peer Communication
While this network organization functions in practice for
client/server-style communication, when the client is behind one or
more levels of NAT and the server is on the public Internet, the lack
Srisuresh & Ford Informational [Page 7]

RFC 5684 Complications from NAT Deployments February 2010
of globally routable addresses for hosts on private networks makes
direct peer-to-peer communication between those hosts difficult. For
example, two private hosts F and H on the network shown above might
"meet" and learn of each other through a well-known server on the
public Internet, such as host A, and desire to establish direct
communication between G and H without requiring A to forward each
packet. If G and H merely learn each other's (private) IP addresses
from a registry kept by A, their attempts to connect to each other
will fail because G and H reside on different private networks.
Worse, if their connection attempts are not properly authenticated,
they may appear to succeed but end up talking to the wrong host. For
example, G may end up talking to host F, the host on private network
3 that happens to have the same private IP address as host H. Host H
might similarly end up unintentionally connecting to host I.
In summary, peer-to-peer communication between hosts on disjoint
private networks 1, 2, 3, and 4 in figure 1 above is a challenge
without the assistance of a well-known server on the public Internet.
However, with assistance from a node in the public Internet, all
hosts on the private networks 1, 2, 3, and 4 in figure 1 above are
able to establish a peer-to-peer-style communication session amongst
themselves as well as with hosts on the public Internet.
3.2. Anomalies of the Multi-Level NAT Network
Even though conventional wisdom would suggest that the network
described above is seriously broken, in practice it still works in
many ways. We break up figure 1 into two sub-figures to better
illustrate the anomalies. Figure 1.1 is the left half of figure 1
and reflects the conventional single NAT deployment that is widely
prevalent in many last-mile locations. The deployment in figure 1.1
is popularly viewed as a pragmatic solution to work around the
depletion of IPv4 address space and is not considered broken. Figure
1.2 is the right half of figure-1 and is representative of the
anomalies we are about to discuss.
Srisuresh & Ford Informational [Page 8]

RFC 5684 Complications from NAT Deployments February 2010
A plug-and-play NAT device provides DHCP service on the LAN attached
to the private interface, and assumes that all private hosts at the
consumer site have DHCP client enabled and are connected to the
single LAN. Consumers need to be aware that all private hosts must
be on a single LAN, with no router in between.
A plug-and-play NAT device also assumes that there is no other NAT
device or DHCP server device on the same LAN at the customer
premises. When there are multiple plug-and-play NAT devices on the
same LAN, each NAT device will offer DHCP service on the same LAN,
and may even be from the same private address pool. This could
result in multiple end nodes on the same LAN ending up with identical
IP addresses and breaking network connectivity.
As it turns out, most consumer deployments have a single LAN where
there they deploy a plug-and-play NAT device and the concerns raised
above have not been an issue in reality.
3.2.2. Unconventional Addressing on NAT Devices
Let us consider the unconventional addressing with NAT-3 and NAT-4.
NAT-3 and NAT-4 are apparently multi-homed on the same subnet through
both their interfaces. NAT-3 is on subnet 10.1.1/24 through its
external interface facing NAT-2, as well as through its private
interface facing clients host F and host G. Likewise, NAT-4 also has
two interfaces on the same subnet 10.1.1/24.
In a traditional network, when a node has multiple interfaces with IP
addresses on the same subnet, it is natural to assume that all
interfaces with addresses on the same subnet must be on a single
connected LAN (bridged LAN or a single physical LAN). Clearly, that
is not the case here. Even though both NAT-3 and NAT-4 have two
interfaces on the same subnet 10.1.1/24, the NAT devices view the two
interfaces as being on two disjoint subnets and routing realms. The
plug-and-play NAT devices are really not multi-homed on the same
subnet as in a traditional sense.
In a traditional network, both NAT-3 and NAT-4 in figure 1.2 should
be incapable of communicating reliably as a transport endpoint with
other nodes on their adjacent networks (e.g., private networks 2 and
3 in the case of NAT-3 and private Networks 2 and 4 in the case of
NAT-4). This is because applications on either of the NAT devices
cannot know to differentiate packets from hosts on either of the
subnets bearing the same IP address. If NAT-3 attempts to resolve
the IP address of a neighboring host in the conventional manner by
broadcasting an Address Resolution Protocol (ARP) request on all of
its physical interfaces bearing the same subnet, it may get a
different response on each of its physical interfaces.
Srisuresh & Ford Informational [Page 11]

RFC 5684 Complications from NAT Deployments February 2010
Even though both NAT-3 and NAT-4 have hosts bearing the same IP
address on the adjacent networks, the NAT devices do communicate
effectively as endpoints. Many of the plug-and-play NAT devices
offer a limited number of services on them. For example, many of the
NAT devices respond to pings from hosts on either of the interfaces.
Even though a NAT device is often not actively managed, many of the
NAT devices are equipped to be managed from the private interface.
This unconventional communication with NAT devices is achievable
because many of the NAT devices conform to REQ-7 of [BEH-UDP] and
view the two interfaces as being on two disjoint routing domains and
distinguish between sessions initiated from hosts on either interface
(private or public).
3.2.3. Multi-Level NAT Translations
Use of a single NAT to connect private hosts to the public Internet
as in figure 1.1 is a fairly common practice. Many consumer NATs are
deployed this way. However, use of multi-level NAT translations as
in figure 1.2 is not a common practice and is not well understood.
Let us consider the conventional single NAT translation in figure
1.1. Because the public and private IP address ranges are
numerically disjoint, nodes on private networks can make use of both
public and private IP addresses when initiating network communication
sessions. Nodes on a private network can use private IP addresses to
refer to other nodes on the same private network, and public IP
addresses to refer to nodes on the public Internet. For example,
host C in figure 1.1 is on private network 1 and can directly address
hosts A, B, and D using their assigned IP addresses. This is in
spite of the fact that hosts A and B are on the public Internet and
host D alone is on the private network.
Next, let us consider the unconventional multi-level NAT topology in
figure 1.2. In this scenario, private hosts are able to connect to
hosts on the public Internet. But, private hosts are not able to
connect with all other private hosts. For example, host F in figure
1.2 can directly address hosts A, B, and G using their assigned IP
addresses, but F has no way to address any of the other hosts in the
diagram. Host F in particular cannot address host E by its assigned
IP address, even though host E is located on the immediately
"upstream" private network through which F is connected to the
Internet. Host E has the same IP address as host G. Yet, this
addressing is "legitimate" in the NAT world because the two hosts are
on different private networks.
It would seem that the topology in figure 1.2 with multiple NAT
translations is broken because private hosts are not able to address
each other directly. However, the network is not broken. Nodes on
Srisuresh & Ford Informational [Page 12]

RFC 5684 Complications from NAT Deployments February 2010
any private network have no direct method of addressing nodes on
other private networks. The private networks 1, 2, 3, and 4 are all
disjoint. Hosts on private network 1 are unable to directly address
nodes on private networks 2, 3, or 4 and vice versa. Multiple NAT
translations were not the cause of this.
As described in sections 3.1.1 and 3.1.2, client-server and peer-to-
peer communication can and should be possible even with multi-level
NAT topology deployment. A host on any private network must be able
to communicate with any other host, no matter to which private
network the host is attached or where the private network is located.
Host F should be able to communicate with host E and carry out both
client-server communication and peer-to-peer communication, and vice
versa. Host F and host E form a hairpin session through NAT-2 to
communicate with each other. Each host uses the public endpoint
assigned by the Internet-facing NAT (NAT-2) to address its peer.
When the deployed NAT devices conform to the hairpin translation
requirements in [BEH-UDP], [BEH-TCP], and [BEH-ICMP], peer nodes are
able to connect even in this type of multi-level NAT topologies.
3.2.4. Mistaken End Host Identity
Mistaken end host identity can result in accidental malfunction in
some cases of multi-level NAT deployments. Consider the scenario in
figure 1.3. Figure 1.3 depicts two levels of NATs between an end-
user in private network 3 and the public Internet.
Suppose CheapoISP assigns 10.1.1.11 to its DNS resolver, which it
advertises through DHCP to NAT-3, the gateway for Ann's home. NAT-3
in turn advertises 10.1.1.11 as the DNS resolver to host F
(10.1.1.10) and host G (10.1.1.11) on private network 3. However,
when host F sends a DNS query to 10.1.1.11, it will be delivered
locally to host G on private network 3 rather than CheapoISP's DNS
resolver. This is clearly a case of mistaken identity due to
CheapoISP advertising a server that could potentially overlap with
its customers' IP addresses.
Srisuresh & Ford Informational [Page 13]

RFC 5684 Complications from NAT Deployments February 2010
the corporate network. While doing so, the employee should not
jeopardize the integrity and confidentiality of the corporate network
and the applications running on the network.
IPsec, Layer 2 Tunneling Protocol (L2TP), and Secure Socket Layer
(SSL) are some of the well-known secure VPN technologies used by the
remote access vendors. Besides authenticating employees for granting
access, remote access VPN servers often enforce different forms of
security (e.g., IPsec, SSL) to protect the integrity and
confidentiality of the run-time traffic between the VPN client and
the VPN server.
Many enterprises deploy their internal networks using private address
space as defined in RFC 1918 and use NAT devices to connect to the
public Internet. Further, many of the applications in the corporate
network refer to information (such as URLs) and services using
private addresses in the corporate network. Applications such as the
Network File Systems (NFS) rely on simple source-IP-address-based
filtering to restrict access to corporate users. These are some
reasons why the remote access VPN servers are configured with a block
of IP addresses from the corporate private network to assign to
remote access clients. VPN clients use the virtual IP (VIP) address
assigned to them (by the corporate VPN server) to access applications
inside the corporate network.
Consider the remote access VPN scenario in figure 2 below.
Srisuresh & Ford Informational [Page 15]

RFC 5684 Complications from NAT Deployments February 2010
coincidentally overlaps the corporate network. In this scenario, it
is conventionally problematic for the VPN client to connect to the
server(s) and other hosts at the enterprise.
Nevertheless, despite the direct address overlap, the remote access
VPN connection between the VPN client at the remote site and the VPN
server at the enterprise should remain connected and should be made
to work. That is, the NAT device at the remote site should not
obstruct the VPN connection traversing it. Additionally, the remote
user should be able to connect to any host at the enterprise through
the VPN from the remote desktop.
The following subsections describe the operational details of the
VPN, anomalies with the address overlap, and recommendations on how
best to address the situation.
4.1. Operational Details of Remote AccessVPN Network
As mentioned earlier, in the "de facto" Internet address
architecture, only the nodes on the public Internet have globally
unique IP addresses assigned by the official IP address registries.
Many of the networks in the edges use private IP addresses from RFC
1918 and use NAT devices to connect their private networks to the
public Internet. Many enterprises adapted the approach of using
private IP addresses internally. Employees within the enterprise's
intranet private network are "trusted" and may connect to any of the
internal hosts with minimal administrative or policy enforcement
overhead. When an employee leaves the enterprise premises, remote
access VPN provides the same level of intranet connectivity to the
remote user.
The objective of this section is to provide an overview of the
operational details of a remote access VPN application so the reader
has an appreciation for the problem of remote address space overlap.
This is not a tutorial or specification of remote access VPN
products, per se.
When an employee at a remote site launches his/her remote access VPN
client, the VPN server at the corporate premises demands that the VPN
client authenticate itself. When the authentication succeeds, the
VPN server assigns a Virtual IP (VIP) address to the client for
connecting with the corporate intranet. From this point onwards,
while the VPN is active, outgoing IP packets directed to the hosts in
the corporate intranet are tunneled through the VPN, in that the VPN
server serves as the next-hop and the VPN connection as the next-hop
link for these packets. Within the corporate intranet, the
Srisuresh & Ford Informational [Page 17]

RFC 5684 Complications from NAT Deployments February 2010
outbound IP packets appear as arriving from the VIP address. So, IP
packets from the corporate hosts to the remote user are sent to the
remote user's VIP address and the IP packets are tunneled inbound to
the remote user's PC through the VPN tunnel.
This works well so long as the subnets in the corporate network do
not conflict with subnets at the remote site where the remote user's
PC is located. However, when the corporate network is built using
RFC 1918 private address space and the remote location where the VPN
client is launched is also using an overlapping network from RFC 1918
address space, there can be addressing conflicts. The remote user's
PC will have a conflict in accessing nodes on the corporate site and
nodes at the remote site bearing the same IP address simultaneously.
Consequently, the VPN client may be unable to have full access to the
employee's corporate network and the local network at the remote site
simultaneously.
In spite of address overlap, remote access VPN clients should be able
to successfully establish connections with intranet hosts in the
enterprise.
4.2. Anomalies of the Remote Access VPNs
Even though conventional wisdom would suggest that the remote access
VPN scenario with overlapping address space would be seriously
broken, in practice it still works in many ways. Let us look at some
anomalies where there might be a problem and identify solutions
through which the remote access VPN application could be made to work
even under the problem situations.
4.2.1. Remote Router and DHCP Server Address Conflict
Routing and DHCP service are bootstrap services essential for a
remote host to establish a VPN connection. Without DHCP lease, the
remote host cannot communicate over the IP network. Without a router
to connect to the Internet, the remote host is unable to access past
the local subnet to connect to the VPN server at the enterprise. It
is essential that neither of these bootstrap services be tampered
with at the remote host in order for the VPN connection to stay
operational. Typically, a plug-and-play NAT device at the remote
site provides both routing and DHCP services from the same IP
address.
When there is address overlap between hosts at the corporate intranet
and hosts at the remote site, the remote VPN user is often unaware of
the address conflict. Address overlap could potentially cause the
remote user to lose connectivity to the enterprise entirely or lose
connectivity to an arbitrary block of hosts at the enterprise.
Srisuresh & Ford Informational [Page 18]

RFC 5684 Complications from NAT Deployments February 2010
Consider, for example, a scenario where the IP address of the DHCP
server at the remote site matched the IP address of a host at the
enterprise network. When the remote user's PC is ready to renew the
lease of the locally assigned IP address, the remote user's VPN
client would incorrectly identify the IP packet as being addressed to
an enterprise host and tunnel the DHCP renewal packet over the VPN to
the remote VPN server. The DHCP renewal requests simply do not reach
the DHCP server at the remote site. As a result, the remote PC would
eventually lose the lease on the IP address and the VPN connection to
the enterprise would be broken.
Consider another scenario where the IP address of the remote user's
router overlapped with the IP address of a host in the enterprise
network. If the remote user's PC were to send a ping or some type of
periodic keep-alive packets to the router (say, to test the liveness
of the router), the packets would be intercepted by the VPN client
and simply redirected to the VPN tunnel. This type of unintended
redirection has the twin effect of hijacking critical packets
addressed to the router as well as the host in the enterprise network
(bearing the same IP address as the remote router) being bombarded
with unintended keep-alive packets. Loss of connectivity to the
router can result in the VPN connection being broken.
Clearly, it is not desirable to route traffic directed to the local
router or DHCP server to be redirected to the corporate intranet. A
VPN client on a remote PC should be configured such that IP packets
whose target IP address matches any of the following are disallowed
to be redirected over the VPN:
a) IP address of the VPN client's next-hop router, used to access the
VPN server.
b) IP address of the DHCP server, providing address lease on the
remote host network interface.
Recommendation-2: A VPN client on a remote PC should be configured
such that IP packets whose target IP address matches *any* of (a) or
(b) are disallowed to be redirected over the VPN:
a) IP address of the VPN client's next-hop router, used to access the
VPN server.
b) IP address of the DHCP server, providing address lease on the
remote host network interface.
Srisuresh & Ford Informational [Page 19]

RFC 5684 Complications from NAT Deployments February 2010
4.2.2. Simultaneous Connectivity Conflict
Ideally speaking, it is not desirable for the corporate intranet to
conflict with any of the hosts at the remote site. As a general
practice, if simultaneous communication with end hosts at the remote
location is important, it is advisable to disallow access to any
corporate network resource that overlaps the client's subnet at the
remote site. By doing this, the remote user is able to connect to
all local hosts simultaneously while the VPN connection is active.
Some VPN clients allow the remote PC to access the corporate network
over VPN and all other subnets directly without routing through the
VPN. Such a configuration is termed as "Split VPN" configuration.
"Split VPN" configuration allows the remote user to run applications
requiring communication with hosts at the remote site or the public
Internet, as well as hosts at the corporate intranet, unless there is
address overlap with the remote subnet. Applications needing access
to the hosts at the remote site or the public Internet do not
traverse the VPN, and hence are likely to have better performance
when compared to traversing the VPN. This can be quite valuable for
latency-sensitive applications such as Voice over IP (VoIP) and
interactive gaming. If there is no overriding security concern to
directly accessing hosts at the remote site or the public Internet,
the VPN client on remote PC should be configured in "Split VPN" mode.
If simultaneous connectivity to hosts at the remote site is not
important, the VPN client may be configured to direct all
communication traffic from the remote user to the VPN. Such a
configuration is termed as "Non-Split VPN" configuration. "Non-Split
VPN" configuration ensures that all communication from the remote
user's PC traverses the VPN link and is routed through the VPN
server, with the exception of traffic directed to the router and DHCP
server at the remote site. No other communication takes place with
hosts at the remote site. Applications needing access to the public
Internet also traverse the VPN. If the goal is to maximize the
security and reliability of connectivity to the corporate network,
the VPN client on remote PC should be configured in "Non-Split VPN"
mode. "Non-Split VPN" configuration will minimize the likelihood of
access loss to corporate hosts.
Recommendation-3: A VPN client on a remote PC should be configured in
"Non-Split VPN" mode if the deployment goal is (a), or in "Split VPN"
mode if the deployment goal is (b):
a) If the goal is to maximize the security and reliability of
connectivity to the corporate network, the VPN client on the
remote PC should be configured in "Non-Split VPN" mode. "Non-
Split VPN" mode ensures that the VPN client directs all traffic
Srisuresh & Ford Informational [Page 20]

RFC 5684 Complications from NAT Deployments February 2010
from the remote user to the VPN server (at the corporate site),
with the exception of traffic directed to the router and DHCP
server at the remote site.
b) If there is no overriding security concern to directly accessing
hosts at the remote site or the public Internet, the VPN client on
the remote PC should be configured in "Split VPN" mode. "Split
VPN" mode ensures that only the corporate traffic is directed over
the VPN. All other traffic does not have the overhead of
traversing the VPN.
4.2.3. VIP Address Conflict
When the VIP address assigned to the VPN client at the remote site is
in direct conflict with the IP address of the existing network
interface, the VPN client might be unable to establish the VPN
connection.
Consider a scenario where the VIP address assigned by the VPN server
directly matched the IP address of the networking interface at the
remote site. When the VPN client on the remote host attempts to set
the VIP address on a virtual adapter (specific to the remote access
application), the VIP address configuration will simply fail due to
conflict with the IP address of the existing network interface. The
configuration failure in turn can result in the remote access VPN
tunnel not being established.
Clearly, it is not advisable to have the VIP address overlap the IP
address of the remote user's existing network interface. As a
general rule, it is not advisable for the VIP address to overlap any
IP address in the remote user's local subnet, as the VPN client on
the remote PC might be forced to respond to ARP requests on the
remote site and the VPN client might not process the handling of ARP
requests gracefully.
Some VPN vendors offer provisions to detect conflict of VIP addresses
with remote site address space and switch between two or more address
pools with different subnets so the VIP address assigned is not in
conflict with the address space at remote site. Enterprises
deploying VPNs that support this type of vendor provisioning are
advised to configure the VPN server with a minimum of two distinct IP
address pools. However, this is not universally the case.
Alternately, enterprises may deploy two or more VPN servers with
different address pools. By doing so, a VPN client that detects
conflict of a VIP address with the subnet at the remote site will
have the ability to switch to an alternate VPN server that will not
conflict.
Srisuresh & Ford Informational [Page 21]

RFC 5684 Complications from NAT Deployments February 2010
Recommendation-4: Enterprises deploying remote access VPN solutions
are advised to adapt a strategy of (a) or (b) to avoid VIP address
conflict with the subnet at the remote site.
a) If the VPN server being deployed has been provisioned to configure
two or more address pools, configure the VPN server with a minimum
of two distinct IP address pools.
b) Deploy two or more VPN servers with distinct IP address pools. By
doing so, a VPN client that detects conflicts of VIP addresses
with the subnet at the remote site will have the ability to switch
to an alternate VPN server that will not conflict.
4.2.4. Mistaken End Host Identity
When "Split VPN" is configured on the VPN client on a remote PC,
there can be a potential security threat due to mistaken identity.
Say, a certain service (e.g., SMTP mail service) is configured on
exactly the same IP address on both the corporate site and the remote
site. The user could unknowingly be using the service on the remote
site, thereby violating the integrity and confidentiality of the
contents relating to that application. Potentially, remote user mail
messages could be hijacked by the ISP's mail server.
Enterprises deploying remote access VPN servers should allocate
global IP addresses for the critical servers the remote VPN clients
typically need to access. By doing this, even if most of the private
corporate network uses RFC 1918 address space, this will ensure that
the remote VPN clients can always access the critical servers
regardless of the private address space used at the remote attachment
point. This is akin to Recommendation-1 provided in conjunction with
multi-level NAT deployments.
Recommendation-5: When "Split VPN" is configured on a VPN client of a
remote PC, enterprises deploying remote access VPN servers are
advised to assign global IP addresses for the critical servers the
remote VPN clients are likely to access.
5. Summary of Recommendations
NAT vendors are advised to refer to the BEHAVE protocol documents
([BEH-UDP], [BEH-TCP], and [BEH-ICMP]) for a comprehensive list of
conformance requirements for NAT devices.
Srisuresh & Ford Informational [Page 22]

RFC 5684 Complications from NAT Deployments February 2010
The following is a summary of recommendations to support the
unconventional NAT topologies identified in this document. The
recommendations are deployment-specific and addressed to the
personnel responsible for the deployments. These personnel include
ISP administrators and enterprise IT administrators.
Recommendation-1: ISPs, using NAT devices to provide connectivity to
customers, should assign non-overlapping addresses to servers
advertised to customers. One way to do this would be to assign
global addresses to advertised servers.
Recommendation-2: A VPN client on a remote PC should be configured
such that IP packets whose target IP address matches *any* of (a) or
(b) are disallowed to be redirected over the VPN:
a) IP address of the VPN client's next-hop router, used to access the
VPN server.
b) IP address of the DHCP server, providing address lease on the
remote host network interface.
Recommendation-3: A VPN client on a remote PC should be configured in
"Non-Split VPN" mode if the deployment goal is (a), or in "Split VPN"
mode if the deployment goal is (b):
a) If the goal is to maximize the security and reliability of
connectivity to the corporate network, the VPN client on the
remote PC should be configured in "Non-Split VPN" mode. "Non-
Split VPN" mode ensures that the VPN client directs all traffic
from the remote user to the VPN server (at the corporate site),
with the exception of traffic directed to the router and DHCP
server at the remote site.
b) If there is no overriding security concern to directly accessing
hosts at the remote site or the public Internet, the VPN client on
the remote PC should be configured in "Split VPN" mode. "Split
VPN" mode ensures that only the corporate traffic is directed over
the VPN. All other traffic does not have the overhead of
traversing the VPN.
Recommendation-4: Enterprises deploying remote access VPN solutions
are advised to adapt a strategy of (a) or (b) to avoid VIP address
conflict with the subnet at the remote site.
a) If the VPN server being deployed has been provisioned to configure
two or more address pools, configure the VPN server with a minimum
of two distinct IP address pools.
Srisuresh & Ford Informational [Page 23]

RFC 5684 Complications from NAT Deployments February 2010
b) Deploy two or more VPN servers with distinct IP address pools. By
doing so, a VPN client that detects conflicts of VIP addresses
with the subnet at the remote site will have the ability to switch
to an alternate VPN server that will not conflict.
Recommendation-5: When "Split VPN" is configured on a VPN client of a
remote PC, enterprises deploying remote access VPN servers are
advised to assign global IP addresses for the critical servers the
remote VPN clients are likely to access.
6. Security Considerations
This document does not inherently create new security issues.
Security issues known to DHCP servers and NAT devices are applicable,
but not within the scope of this document. Likewise, security issues
specific to remote access VPN devices are also applicable to the
remote access VPN topology, but not within the scope of this
document. The security issues reviewed here only those relevant to
the topologies described in sections 2 and 3, specifically as they
apply to private address space overlap in the topologies described.
Mistaken end host identity is a security concern present in both
topologies discussed. Mistaken end host identity, described in
sections 2.2.4 and 3.2.4 for each of the topologies reviewed,
essentially points the possibility of application services being
hijacked by the wrong application server (e.g., Mail server).
Security violation due to mistaken end host identity arises
principally due to critical servers being assigned RFC 1918 private
addresses. The recommendation suggested for both scenarios is to
assign globally unique public IP addresses for the critical servers.
It is also recommended in section 2.1.2 that applications adapt end-
to-end authentication and not depend on source IP address for
authentication. Doing this will thwart connection hijacking and
denial-of-service attacks.
7.Acknowledgements
The authors wish to thank Dan Wing for reviewing the document in
detail and making helpful suggestions in reorganizing the document
format. The authors also wish to thank Ralph Droms for helping with
rewording the text and Recommendation-1 in section 3.2.4 and Cullen
Jennings for helping with rewording the text and Recommendation-3 in
section 4.2.2.
Srisuresh & Ford Informational [Page 24]